Detection elements for use with pressure detectors have heretofore been known which utilize the piezo-resistance effect of Si and a piezoelectric material such as rock crystal, PZT, and so forth.
Among these conventional elements, an element relying upon the piezo-resistance effect of Si cannot be used at a very high temperature since its effective temperature range is limited. Another element utilizing rock crystal is capable of withstanding relatively high temperatures, but it is inferior in detection sensitivity since its piezoelectric constant is not very large. In the case of the PZT piezoelectric element, its effective temperature range is limited, so that it is still difficult to use the element at high temperatures and its hysteresis with respect to pressure is disadvantageously large.
To overcome these problems, the specification of Japanese Patent Examined Publication No. 16379/1980 has proposed a piezoelectric ceramic composition which eliminates the above noted disadvantages by using a sintered body having a bismuth layer structure of PbBi.sub.4 Ti.sub.4 O.sub.15 in which the temperature-dielectric constant characteristic is relatively stable up to a high temperature region, and a small amount of MnO is added to the sintered body with a view to further improving the temperature characteristics.
However, it has been found during the experiments and studies conducted by the inventors that the abovedescribed conventional piezoelectric ceramic composition composed of MnO added to PbBi.sub.4 Ti.sub.4 O.sub.15 involves disadvantage in that the temperature-dependent change rate of the dielectric constant of a ferroelectric substance having a bismuth layer structure exceeds 10% within the range, for example, between a normal temperature (about 25.degree. C.) and 150.degree. C. even if MnO is added, that is, the dielectric constant still changes greatly in relation to temperature. In general, the piezoelectric constant d.sub.33 of a piezoelectric material is represented by the following equation ##EQU1## In this equation, it has been found that the piezoelectric constant d.sub.33 is most seriously affected by the dielectric constant .epsilon..sup.T.sub.33 among the parameters contained on the right side of the equation, i.e., K.sub.33 (coupling factor), .epsilon..sup.T.sub.33 (dielectric constant) and Y.sup.E.sub.33 (Young's modulus).
Hence, the above-mentioned prior-art piezoelectric ceramic composition still cannot prevent significant changes in the piezoelectric constant d.sub.33 due to temperature changes, and thus a compensation circuit is indispensable in order that these changes can be compensated for. As a result, another problem is raised in that the compensation circuit must be made complicated under circumstances in which temperature changes to a remarkable extent.